In a motor system, an excessive current higher than that under normal operating condition flows in motor windings due to dielectric breakdown, overload, forced constraint, or the like. If such an excessive current flows, temperatures of the windings rise and exceed a heat-withstanding temperature of insulating coating of the windings, dielectric breakdown occurs, the windings short out. This further leads to abnormal operations of an apparatus connected to the motor.
To prevent such overheating of the windings, temperature protective devices such as a thermistor and a thermal fuse are attached and the temperature rise is suppressed by limiting currents flowing in the windings in response to winding temperatures or cutting off currents when they exceed a predetermined temperature. However, such overheat protection needs addition of a thermistor, a thermal fuse, etc. and addition of a peripheral circuit for detection. This causes increase in cost.
Moreover, as overheat protection techniques of the direct current motor, winding temperatures are estimated from a motor current value squared and a motor resistance value, and the current is limited according to the estimated temperature because the winding temperature is proportional to a product of the motor current value squared by the motor resistance value.
The brushless motor uses three-phase alternating currents. Consequently its accurate state cannot be grasped, except that the sampling period and calculation period of current, voltage, etc. are done at high speed. In a worst case, it is likely that calculation results may be different from actual values depending on the sampling period (for example, only low current values of the alternating waveform are sampled), and hence dielectric breakdown may occur due to a failure of the overheat protection.
For example, supposing that the brushless motor with 16 poles is in rotation at 2000 rpm, a motor current waveform becomes an alternating waveform of a period of 3.75 ms. That is, if this alternating waveform is intended to be sampled accurately and subjected to calculation, it must be performed in a period of a few hundreds of micro seconds or less, which increases a calculation load of a microcomputer. To prevent the calculation load in the microcomputer from increasing, a microcomputer capable of high-speed processing is necessitated. This will result in an increase of cost.
It is also proposed in JP 2003-164185A to realize the overheat protection by using a q-axis current that is a torque component current of the brushless motor, or both the q-axis current and a d-axis current that is an exciting component current while preventing increase in cost and calculation load.
The brushless motor is driven by the three-phase alternating currents. Each of its voltage, current, magnetic flux, etc. is represented by a composite vector that is a vector sum of components generated by alternating components of each phase. When driving the brushless motor, its control is simplified by converting the three-phase alternating currents that necessitate handling these vectors to two-axis direct currents.
This two-axis direct current conversion means conversion whereby a motor having a fixed part and a rotating part is converted to one in an orthogonal coordinate system whose coordinates are both fixed, i.e., a rotating orthogonal coordinate system, and is called d-q conversion. The q-axis is in advance of the d-axis by a phase of π/2 and the d-axis is orientated in a direction of a magnetic flux formed by a field magnet.
In the direct current motor, a field magnet circuit is formed with permanent magnets or by flowing constant field-magnet currents in field-magnet windings. Independently from it, an armature current is supplied in a rotor conductor from the outside, whereby a torque proportional to the armature current can be generated. Therefore, the direct current motor is rotated with the torque proportional to the armature current.
On the other hand, in the brushless motor, the rotor is not electrically connected with the outside. That is, only a primary current flowing in the stator generates both a rotating magnetic field and an induction current equivalent to the armature current. Therefore, the primary current contains both a current (exciting component current) that generates a secondary interlinkage flux crossing a secondary-side rotor and a current (torque component current) that flows in a secondary-side conductor.
A technique of controlling these two currents independently is the d-q conversion described above. With this conversion, if the exciting component current (d-axis current) is controlled to be constant, the torque component current (q-axis current) will be proportional to the torque. Therefore, the use of this q-axis current enables the brushless motor to be vector-controlled as in the case of the direct current motor.
Here, the principle of vector control will be described using FIG. 7. The magnitude and phase of a current determines the torque of the brushless motor, the alternating current motor, or the like. In practice, the current is divided into a current component (magnetic flux current) that forms a magnetic flux in the direction of a main magnetic flux established inside the motor and a current component with a phase advanced by 90° that controls the torque directly (torque current).
The two components are controlled independently. The magnetic flux current and the torque current are defined as a current component that forms a magnetic field in the d-axis direction and a current component that forms a magnetic field in the q-axis direction, respectively. The fact that each of the current, voltage, and magnetic flux is controlled after being divided into a d-axis component and a q-axis component may account for a name of the vector control. These current components can be calculated by the well known three-phase to two-phase conversion based on a rotation angle θ of the main magnetic flux to the stator in the d-q axis coordinate system.
Moreover, in estimation calculation of the winding temperature (electric power), the use of the d-axis current in addition to the q-axis current enables estimation with higher accuracy than that of an estimation only with the q-axis current, although it adds a slight increase in the calculation load.
However, in the above conventional method, because the brushless motor uses the three-phase alternating currents, when the motor is in rotation or when the motor is not in rotation and yet predetermined currents are flowing through it so as to produce a torque (motor lock state), currents applied to phase windings might differ largely among them even if the q-axis current or the d-axis current is the same. The current also might differ depending on a motor locking position (electrical angle).
As a result, in the case where only the q-axis current or the d-axis current is used to estimate the winding temperatures and the overheat protection is performed, estimation errors may occur and excessive overheat protection may be executed, which impedes the production of sufficient torque by the motor. Otherwise, too little overheat protection may result in short circuit of the windings, which leads to abnormal operations of an apparatus connected to the motor.